Post-Tensioned Concrete Slabs-on-Ground

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  • Slab-on-Ground Design

    by Ken BondyUsing the PTI


  • Biggest Single U.S. Market for Post-Tensioning Tendons

    Over 40%Over 40% of all P/T tendons sold in the USA are for residential foundations.58,000 tons58,000 tons of tendons installed in residential foundations in the year 2000 alone.Represents about 220,000 homes in one year.

  • HistoryFirst used the late 1960s in Texas and CaliforniaWidest usage has been in

    Texas (50%)California (32%)Nevada (7%) Louisiana (5%)

    Usage increasing (4%) inArizonaColorado FloridaGeorgia

  • Evolution of Design MethodsEarliest methods (1965) were semi-empirical

    Based upon simplified mathematical modelsAssumed loss of support (Spanability)

    Confirmed by actual performancePTI Method (1980) based upon rigorous mathematical study of soil/structure interaction

    Most comprehensive design method ever developed for behavior of concrete foundations on expansive soils

  • The PTI Design MethodThe PTI Design MethodBased upon a finite element computer model of soil/structure interaction, with research sponsored by PTI and executed at Texas A & M University in late 1970s1st Edition published in 1980, 2nd Edition in 1996Incorporated into model building codes (UBC 1997, IBC 2000)Used to design millions of existing foundations

  • PTI Publications

  • Technical Information Available

  • Structural Function of P/T FoundationActs as a buffer between the soil below and the superstructure above to prevent unacceptable deformations in superstructure.Foundation is designed to resist or span over moisture-induced deformations in the soil below, while still maintaining its top surface within permissible level tolerances.

  • Expansive Soil Swell Modes

    Edge LiftSoils are wetter at slab edge than at any point inside slab edge.

    Center LiftSoils are drier at slab edge than at any point inside slab edge.

  • Soil Model

  • Edge Lift

  • Center Lift (Edge Drop)

  • Geotechnical Engineer Provides Critical Soil Design Parameters

    Edge Moisture Variation Distance eemmThornthwaite Moisture Index (climate)Soil PermeabilityVegetation

    Unrestrained Differential Swell yymmProperties (activity) of clayDepth of clay (active zone)Soil suction

    One set of eemm & yymm values established for each swell mode (edge and center lift)Design cannot be done without these parameters

    3.0 in0.75 inyymm

    5.0 ft2.0 fteemm

    Center Lift

    Edge Lift

  • New Method For Determining em & ymUnder review by PTI Slab-on-Ground Committee.em based upon soil properties:

    Unsaturated diffusion coefficient Soil suction.Soil permeability.Cracks and roots.

    Soil Fabric Factor (roots, cracks, layers).

    Simplified method for determining ym based primarily on soil suction profiles.Considers effect of vertical barriers (cutoff walls).

  • Edge Lift

    All Structural Activity


    Wet to Dry

  • Center Lift

    All Structural Activity


    Dry to Wet

  • Ribbed and Uniform Thickness Foundations

    PTI Design Method based on ribbed foundation system

    Slab thickness t=4 minimumRibs=Grade Beams

    h=t+7 with 12 minimum depth.b=8-14

    Rib spacing S=6 minimum, 17 maximum.Can be converted to uniform (solid) thickness slab

  • Overlapping Rectangles

    Determine preliminary geometry and layout:

    Rib spacingRib sizeSlab thickness

    Divide slab into overlapping rectangles congruent with slab perimeter.

  • Rectangle A

    24 x 42

  • Rectangle B

    16 x 36

  • Design EquationsFor each swell mode (edge or center lift)

    For each direction (Long or Short), use equations to determine:

    Maximum MomentMaximum ShearMaximum Differential deflection

  • Design Based on Uncracked Section

    Effects of cracking studied in detail in original research and subsequent publications available through PTI.Effects of cracking generally inconsequential due to

    Location of shrinkage cracks.Increased soil support after flexural cracking.

  • Allowable Concrete StressesFlexural

    Tension 6fcCompression 0.45fc

    Shearvc = = 1.7fc + 0.2fp

    Differential deflectionL/C

    L=smaller of total slab length or 6.C = coefficient based on superstructure material.

    FlexuralTension 6fcCompression 0.45fc

    Shearvc = = 1.7fc + 0.2fp

    Differential deflectionL/C

    L=smaller of total slab length or 6.C = coefficient based on superstructure material.

  • Typical Slab Layout

  • Typical Details

  • Ribbed Foundation

  • Uniform Thickness Foundation

  • Thank You!

    Slab-on-Ground DesignBiggest Single U.S. Market for Post-Tensioning TendonsHistoryEvolution of Design MethodsThe PTI Design MethodPTI PublicationsTechnical Information AvailableStructural Function of P/T FoundationExpansive Soil Swell ModesSoil ModelEdge LiftCenter Lift (Edge Drop)Geotechnical Engineer Provides Critical Soil Design ParametersNew Method For Determining em & ymEdge LiftCenter LiftRibbed and Uniform Thickness FoundationsOverlapping RectanglesRectangle A24 x 42Rectangle B16 x 36Design EquationsDesign Based on Uncracked SectionAllowable Concrete StressesTypical Slab LayoutTypical DetailsRibbed FoundationUniform Thickness FoundationThank You!